1. Field of the Invention
The present invention relates generally to computers and computer networks, and more particularly, to data communication protocols for processing information communicated between hosts such as computers connected to a network.
2. Description of the Related Art
The advantages of network computing are increasingly evident. The convenience and efficiency of providing information, communication or computational power to individuals at their personal computer or other end user devices has led to rapid growth of such network computing, including internet as well as intranet systems and applications.
As is well known, most network computer communication is accomplished with the aid of a layered software architecture for moving information between host computers connected to the network. The layers help to segregate information into manageable segments. The general functions of each layer may be based on a reference model—Open Systems Interconnection (OSI). OSI sets forth seven processing layers through which information may pass when received by a host in order to be presentable to an end user. Similarly, transmission of information from a host to the network may pass through those seven processing layers in reverse order. Each step of processing and service by a layer may include copying the processed information. Another reference model that is widely implemented, called TCP/IP (TCP stands for transport control protocol, while IP denotes Internet protocol) essentially employs five of the seven layers of OSI.
Networks may include, for instance, a high-speed bus such as an Ethernet connection or an Internet connection between disparate local area networks (LANs), each of which includes multiple hosts, or any of a variety of other known means for data transfer between hosts. According to the OSI standard, physical layers are connected to the network at respective hosts, the physical layers providing transmission and receipt of raw data bits via the network. A data link layer is serviced by the physical layer of each host, the data link layers providing frame division and error correction to the data received from the physical layers, as well as processing acknowledgment frames sent by the receiving host. A network layer of each host is serviced by respective data link layers, the network layers primarily controlling size and coordination of subnets of packets of data.
A transport layer is serviced by each network layer and a session layer is serviced by each transport layer within each host. Transport layers accept data from their respective session layers and split the data into smaller units for transmission to the other host's transport layer, which concatenates the data for presentation to respective presentation layers. Session layers allow for enhanced communication control between the hosts. Presentation layers are serviced by their respective session layers, the presentation layers translating between data semantics and syntax which may be peculiar to each host and standardized structures of data representation. Compression and/or encryption of data may also be accomplished at the presentation level. Application layers are serviced by respective presentation layers, the application layers translating between programs particular to individual hosts and standardized programs for presentation to either an application or an end user. The TCP/IP standard includes the lower four layers and application layers, but integrates the functions of session layers and presentation layers into adjacent layers. Generally speaking, application, presentation and session layers are defined as upper layers, while transport, network and data link layers are defined as lower layers.
The rules and conventions for each layer are called the protocol of that layer, and since the protocols and general functions of each layer are roughly equivalent in various hosts, it is useful to think of communication occurring directly between identical layers of different hosts, even though these peer layers do not directly communicate without information transferring sequentially through each layer below. Each lower layer performs a service for the layer immediately above it to help with processing the communicated information. Each layer saves the information for processing and service to the next layer. Due to the multiplicity of hardware and software architectures, systems and programs commonly employed, each layer is necessary to insure that the data can make it to the intended destination in the appropriate form, regardless of variations in hardware and software that may intervene.
In preparing data for transmission from a first to a second host, some control data is added at each layer of the first host regarding the protocol of that layer, the control data being indistinguishable from the original (payload) data for all lower layers of that host. Thus, an application layer attaches an application header to the payload data and sends the combined data to the presentation layer of the sending host, which receives the combined data, operates on it and adds a presentation header to the data, resulting in another combined data packet. The data resulting from combination of payload data, application header and presentation header is then passed to the session layer, which performs required operations including attaching a session header to the data and presenting the resulting combination of data to the transport layer. This process continues as the information moves to lower layers, with a transport header, network header and data link header and trailer attached to the data at each of those layers, with each step typically including data moving and copying, before sending the data as bit packets over the network to the second host.
The receiving host generally performs the converse of the above described process, beginning with receiving the bits from the network, as headers are removed and data processed in order from the lowest (physical) layer to the highest (application) layer before transmission to a destination of the receiving host. Each layer of the receiving host recognizes and manipulates only the headers associated with that layer, since to that layer the higher layer control data is included with and indistinguishable from the payload data.
Due to the number of protocol layers through which the data is processed prior to its transmission as bit packets over the network, debugging the various protocol layers in the event that the data is not successfully transmitted is difficult, if not impossible. Many factors may have hindered the data from being successfully transmitted, e.g., one of the protocol layers may have erroneously discarded the data. This difficulty is further accentuated by the fact that the higher protocol layers have no knowledge of how data is processed at the lower protocol layers. Consequently, when data is lost, it is generally difficult to debug the process.
Therefore, there exists a need for a method that facilitates tracing the data as it is processed through the various protocol layers.